The frustrated Lewis pair (FLP) adducts PB{ECl2} (PB = iPr2P(C6H4)BCy2; Cy = cyclohexyl; E = Si, Ge) were used to access a bis(alkynyl)-functionalized silylene and a germylene; the goal behind preparing these species was to obtain new unsaturated main group polymers [E(CCSiMe3)2]n upon heating. While the silylene adduct PB{Si(CCSiMe3)2} was stable up to 150 °C, the heavier element congener PB{Ge(CCSiMe3)2} underwent a complicated rearrangement process accompanied by Cy-group migration and Ge(II)-alkyne coordination. Density functional theory computations were performed to understand the mechanistic pathway for the unusual rearrangement of PB{Ge(CCSiMe3)2}.